Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input driven by an engine crankshaft, often via a launch device such as a torque converter, and an output driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. In front wheel drive vehicles with transverse mounted engines, the engine crankshaft axis is typically offset from the axle axis.
Hybrid electric transmissions further reduce fuel consumption by including one or more reversible electric machines and some type of electrical energy storage such as a battery. Hybrid electric transmissions improve fuel efficiency is several ways. Most internal combustion engines are most efficient when operated at relatively high power settings. A hybrid electric transmission permits operating the engine part of the time at a higher power level than needed for propulsion while storing the excess power in the battery. Then, at other times, the engine is shut off and the vehicle is propelled using the stored energy. Although the engine generates the same amount of total energy, it operates at a higher average efficiency. Also, when the brakes are applied, the reversible electrical machine may capture the vehicle kinetic energy and store it in the battery for later use for propulsion. When the vehicle is stationary and therefore does not require propulsion, the engine can be shut off to eliminate the fuel that would otherwise be used to maintain an idle speed. The electric motor provides the ability to propel the vehicle with the engine off and to rapidly restart the engine when necessary.
A hybrid electric transmission layout is shown in FIG. 1. Input shaft 10 is driven by the engine crankshaft. Output shaft 12 drives the vehicle wheels. The transmission components are supported within transmission housing 14. A first reversible electric machine 16 includes a stator 18 fixed to housing 14 and a rotor 20. A second reversible electric machine 22 includes a stator 24 fixed to housing 14 and a rotor 26. Although both electric machines are capable of both converting electrical power into mechanical power (motoring) and converting mechanical power into electrical power (generating), the first electric machine may be referred to as the generator and the second electric machine may be referred to as the motor. A simple planetary gear set 28 includes a sun gear 30 fixedly coupled to rotor 20, a ring gear 32 fixedly coupled to output shaft 12, a carrier 34 fixedly coupled to input shaft 10, and a set of planet gears 36 supported to rotate with respect to carrier 34 and to mesh with both sun gear 30 and ring gear 32. Gear set 28 divides the torque from the internal combustion engine between the output shaft and the first rotor. Ravigneux gear set 38 includes sun gear 40 fixedly coupled to second rotor 26, ring gear 42, carrier 44 fixedly coupled to output shaft 12, and a set of long planet gears 46 supported for rotation with respect to carrier 44 and to mesh with both sun gear 40 and ring gear 42. Ravigneaux gear set 38 further includes sun gear 48 and a set of short planet gears that each mesh with sun gear 48 and one of the long planet gears 46. Brake 52 selectively holds ring gear 42 against rotation. When brake 52 is engaged, a low underdrive speed relationship is established between rotor 26 and output shaft 12. Brake 54 selectively holds sun gear 48 against rotation. When brake 54 is engaged, a high underdrive speed relationship is established between rotor 26 and output shaft 12.
To launch the vehicle in a forward direction, brake 52 is engaged. Whenever the vehicle is stationary, output shaft 12 is also stationary. Forward rotation of input shaft 10 when output shaft 12 is stationary or moving slowly causes rotor 20 to rotate in a forward direction at a speed higher than the speed of input shaft 10. Electrical voltage in stator 18 is controlled such that rotor 20 exerts torque in the negative direction providing a reaction torque. Consequently, ring 32 exerts a forward torque proportional to the input shaft torque on output shaft 12. Since the rotor is exerting torque in the opposite direction of its rotation, electrical machine 16 acts as a generator producing electrical power. Electrical voltage in stator 24 is controlled to utilize that electrical power to exert a positive torque on sun gear 40. With brake 52 engaged, rotor 26 rotates in the same direction as output shaft 12 at a fixed multiple of the speed of output shaft 12. The speed ratio is determined by the tooth counts on sun gear 40 and ring gear 42. Disregarding motor efficiency, for a given amount of electrical power, the torque generated by rotor 26 is inversely proportional to rotor speed. Gear set 38 multiplies the torque produced by rotor 26 by the speed ratio and transmits that torque to output shaft 12. The total torque on output shaft 12 is the sum of the torque exerted by ring gear 32 and the torque exerted by carrier 44.
As the vehicle speed increases, the speed of rotor 26 may exceed its maximum operating speed. The transmission may shift into a high speed operating mode by gradually releasing brake 52 while gradually engaging brake 54. With brake 54 engaged, the rotor 26 rotates at a lower multiple of output shaft 12 speed than when brake 52 is engaged. The speeds of the input shaft 12 and rotor 20 are not impacted by this shift. At a sufficiently high output shaft speed relative to the input shaft speed, rotor 20 begins to rotate in the opposite direction. In order to continue providing a reaction to engine torque, electrical machine 16 must act as a motor. The electrical power required by electrical machine 16 must either come from the battery or by operating electrical machine 22 as a generator. In this circumstance, power circulates from machine 16, to output shaft 12, to machine 22, and back to machine 16. Such circulation of power results in reduced overall power transfer efficiency.
Brake 52 is also engaged to launch the vehicle in a reverse direction. Stator 24 is controlled such that rotor 26 exerts torque in a reverse direction which is transmitted to output shaft 12. If the battery state of charge is sufficient, the battery may provide the power required by machine 22 in reverse. If the battery state of charge is too low or some other circumstance prevents using battery power, then the power must be produced by operating machine 16 as a generator. As when launching in a forward direction, using machine 16 as a generator causes ring gear 32 to exert torque in the positive direction on output shaft 12. This torque opposes the negative torque exerted by machine 22 through gear set 38 reducing the total output shaft torque. Consequently, machine 22 must produce even more torque. Since the maximum torque capability of machine 22 is limited, the maximum reverse torque capability of the transmission in reverse is considerably less than the maximum capability in forward, especially when the battery state of charge is low.